Graphics controller circuits which generate display signals to display an image corresponding to input display data are known in the prior art. FIG. 1 is a block diagram of computer system 100 comprising one such graphics controller circuit 120. Graphics controller circuit 120 may generate display signals over display bus 123 to display an image corresponding to input display data on display unit 130.
Display data may include different types of data such as video data, graphics data, and text data. Video data may comprise, for example, a television signal encoded in RGB 888 format or in a monochrome pixel data format. Such video data may correspond to display on individual pixels on display unit 130. Graphics controller circuit 120 may receive such video data from an external device (not shown) over video port 151, or from host 110 over system bus 112.
Text data may comprise alpha-numeric characters represented by a predetermined number of bits of input display data. Graphics controller circuit 110 may map each alpha-numeric character to a predetermined pixel data pattern, and generate display signals corresponding to such pixel data pattern. Graphics data may comprise pixel data corresponding to each pixel on display unit 130.
Graphics controller circuit 120 may perform several pre-processing steps on input display data, and provide the output of such pre-processing steps as operands for a raster operation. Graphics controller 120 may provide the output pixel data of such raster operations as input to an additional raster operation, or generate display signals from the pixel data generated from such raster operations.
A raster operation may comprise a plurality of operands, and may be specified by host 110 over system bus 112. For example, host 110 may request graphics controller circuit 120 to perform a logical OR raster operation of two operands, with each operand comprising a portion of the input display data. Graphics controller circuit 120 may generate display pixel data by performing such a raster operation, and generate display signals from such display pixel data.
The raster operations specified by host 110 may further include a transparency operation. In a transparency operation, a portion of the pixel data may be coded in a predetermined color, and such portion may be replaced with another portion of the pixel data. The two portions of the pixel data may correspond to two images. Such a transparency operation may be provided to selectively superimpose one image over another image.
For example, in a weather report review of a television broadcast, a weather reporter may be located in front of a blue background. By using a transparency operation, the blue portion of the image may be superimposed by an image of a weather map to give the perception that the weather reporter is in front of the weather map. Such a background color may be hereinafter referred to as a background color for a transparency operation.
The pre-processing steps performed by graphics controller circuit 120 prior to executing a raster operation may include several operations. For example, such operations may include converting a monochrome pixel data to a color pixel data. The operations may also include shifting pixel data to achieve a required alignment of a pixel data within a memory word. Such shifting may be required, for example, as pixel data of an operand may be stored beginning from an intermediate position within a memory word.
A prior art graphics controller 120 may employ a single pipe-lined architecture to perform a set of pre-processing steps for generating the operands of a raster operation. In a single pipe-lined architecture, graphics controller circuit 120 may comprise sub-circuits which may operate concurrently to perform individual operations comprised in the pre-processing steps. By performing operations concurrently, graphics controller circuit 120 may enhance an output rate of pixel data generated for display on display unit 130. Graphics controller circuit 120 of the prior art may include only a single pipe-line to conserve silicon space.
One problem with graphics controller circuit 120 employing only a single pipe-line for generating operands for raster operations is that the pixel data output rate from raster operations may be insufficient to maintain a required refresh rate on display unit 130. The pixel data output rate may be insufficient, for example, as a single pipe-line in graphics controller 120 may generate sequentially pixel data corresponding to each operand of a raster operation.